Running a Perl program via cron
March 8, 2019
If you want to run the tweetfile.pl Perl program I wrote at regular intervals, say three times a day, you can use
First, write your current crontab to a file as follows:
crontab -l >crontab.txt
If this is the first time you use cron the file might be empty. A crontab file has per entry you want to run at specific times a single line as follows:
* * * * * command to be executed ^ ^ ^ ^ ^ | | | | | | | | | +-- day of week (0 - 6, Sunday = 0) | | | +------ month (1 - 12) | | +---------- day of month (1 - 31) | +-------------- hour (0 - 23) +------------------ min (0 - 59)
The rest of this post assumes you want to run the
tweetfile.pl Perl program.
Crontab with system Perl
If you installed
tweetfile.pl on a system running Ubuntu and installed the required modules using
apt-get you are using the system's version of Perl.
If you want to tweet automatically at, say 21:17, use the following entry:
17 21 * * * cd $HOME/twitter-tools && perl tweetfile.pl --quiet --conf john_bo kma.conf --tweets john_bokma-tweets.txt
Note that the hour comes second and also note that this must be a single long line.
In this example the
tweetfile.pl program is located in the
twitter-tools directory located in the home directory.
You can activate the new crontab using:
Note run the above each time you change your
tweetfile.pl program generates any output
cron will email this output. However, on a default installation of Ubuntu desktop there is no Mail Transport Agent (MTA) installed and
cron will log an additional message to the system log
Mar 8 21:17:01 ecce CRON: (john) CMD (cd $HOME/twitter-tools && perl twee tfile.pl --dry-run --conf john_bokma.conf --tweets john_bokma-tweets.txt) Mar 8 21:17:02 ecce CRON: (CRON) info (No MTA installed, discarding outpu t)
When all things work correctly, it's not really needed that the Perl program
outputs any information, hence why I provided the
--quiet option. But what if things don't work?
Debugging your crontab
If the program you want to run via cron doesn't work as expected, in this case no tweet is posted, how to figure out what went wrong? First, check your system's log file:
grep CRON /var/log/syslog
Does it show an entry at the time you configured? If not, you might have made a mistake I made a few times: swapped minutes and hours. Double check your cron file, remember minutes go first.
If cron complains
No MTA installed, discarding output you can either install an MTA like
postfix or easier, log the output to a file. To do the latter put the following after your command line in
The complete entry becomes:
17 21 * * * cd $HOME/twitter-tools && perl tweetfile.pl --quiet --conf john_bo kma.conf --tweets john_bokma-tweets.txt >>$HOME/cronlog.txt 2>&1
This creates a file in your home directory named cronlog.txt and appends (
>>) both the stderr and stdout to it. Watch this file using:
tail -F cronlog.txt
Both errors and normal output should show up in this file.
If you try to run a program that has no
--quiet option and unwanted output is generated you can silence it by sending the output to
/dev/null, i.e. use
This discards both errors and normal output and should stop
cron from complaining in the system's log about an MTA not being installed.
Crontab with Perlbrew
If you use Perlbrew things are slightly more complicated. You have to run your program via
perlbrew needs to know the location of its home directory. This is achieved by setting the variable
PERLBREW_ROOT before the call of
41 20 * * * cd $HOME/Amber/in-house/projects/twitter-tools && PERLBREW_ROOT=$HOM E/.my-local/perl5 $HOME/.my-local/perl5/bin/perlbrew exec --quiet --with threade d-perl-5.22.0 perl tweetfile.pl --quiet --conf john_bokma.conf --tweets john_bok ma-tweets.txt
Note that I have my Perlbrew root in a non-standard location. Moreover, both
perlbrew and the
tweetfile.pl program have a
--quiet option to prevent them from sending output.